How to choose the right retaining wall for your project

How to choose the right
retaining wall for your project
There is much to consider when choosing the right retaining wall for your application.
We have drawn on years of experience in the earth stabilisation business and put together a short checklist of seven
fundamental choice parameters.
This document is by no means definitive, but it aims to equip you with sufficient information to guide you through the initial
phase of the decision making process.
The seven fundamental choice parameters:
1. The type of retaining wall
Retaining walls can be broadly divided into four main categories — namely, gravity walls, cantilever walls, anchored
walls, and reinforced soil or nailed walls.
Each type of retaining wall has its own particular characteristics, which make it more suitable for certain applications
than others.
Here is a brief overview of the four categories of retaining walls:
a. Gravity walls
These are arguably the oldest and most frequently used retaining walls. They are often made from stone or other
heavy materials, and rely on the weight of their own mass to resist pressures from the retained material.
Gravity walls should be a minimum of 50 to 60% as thick as the height of the wall. They may have to be larger if there
is a slope or surcharge on the wall.
Modern gravity walls include concrete crib walls, gabions, boulders, and large, precast concrete blocks.
Gabions are a type of soil strengthening, consisting of wire mesh cages into which roughly cut stone or other material
is filled. Gabions work to reduce internal movement and erosive forces.
b. Cantilever walls
Before the introduction of modern reinforced-soil gravity walls, cantilevered walls were the most common type of taller
retaining wall. This type of wall uses much less material than a traditional gravity wall.
Cantilevered walls are made from a relatively thin stem of steel-reinforced, cast-in-place concrete or mortared masonry,
fixed at one end, usually by way of a simple, embedment or cantilever foundation (often in the shape of an inverted T).
These rigid concrete footings must be positioned into firm suitable foundations.
The wall operates like a beam — cantilevering the load to a large, fixed structural base — converting horizontal
pressures from behind the wall into vertical pressures onto the ground below.
Sometimes cantilevered walls are buttressed on the front, or include a counterfeit on the back, to improve their
stability against heavy loads.
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Buttresses are short wing-like walls at right angles to the main trend of the wall.
Typical cantilever walls include reinforced concrete, or concrete-filled blockwork, concrete or timber sleeper walls,
concrete, steel or timber sheet pile, or contiguous piling.
c. Anchored walls
These walls are pinned both top and bottom using cables, or other stays, which are anchored in the rock or soil
behind it.
Anchors are driven into the material and then expanded at the end of the cable, either by mechanical means or by
injecting pressurised concrete into the hole. They concrete expands to form a bulb in the soil.
The wall may be embedded at the base and tied to a slab at the top or to a “deadman anchor” — a concrete structure
which is driven into the ground or anchored to the earth with sufficient resistance. The horizontal cable, rod or helical
anchor, and deadman structure resists forces that would otherwise cause the wall to become unstable.
This method, though technically complex, is useful where high loads are expected, or where the wall itself has to be
slender and would be too weak without anchoring.
d. Reinforced soil or nailed walls
These systems do not simply consist of the wall itself, but make use of reinforcing grids or straps to contain and
stabilise the slope.
The traction-resistant reinforcement elements change the nature of the soil mass and reduce the earth pressure acting
on the wall.
In mechanically stabilised earth (MSE) walls the soil is artificially reinforced with layered horizontal mats (geosynthetics).
These mats provide additional internal shear resistance beyond that of simple gravity retaining structures. Layered steel
straps may also be used.
Reinforcing layers are attached to outer facing walls (typically segmental retaining walls). The wall face is usually made
of precast concrete units that can tolerate some differential movement.
When the reinforced soil mass is sufficiently large to retain the pressure from the soil behind it, it works with the wall
to provide stability.
Nailed walls make use of slender elements — normally steel reinforcing bars — which are inserted into pre-drilled hole
and grouted into place. The reinforcing bars are usually installed untensioned at a slight downward angle. Isolated soil
nail heads or a rigid (or flexible) facing (often of sprayed concrete) may be used on the surface of the wall.
Typical reinforced soil or nailed walls include concrete or steel panel-faced R.E. walls, segmental R.E. walls, wire mesh
or fabric-faced R.E. walls and shotcrete-faced nailed walls.
2.The material to be retained
Important questions to ask upfront include:
• Will the wall be retaining a cut face or fill material?
• If it is fill material, will it be controlled fill?
• If it is a cut face, what type of ground is it?
Generally, building a wall to retain fill material allows for a wider choice of wells than a wall designed to retain cut faces.
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Cut faces are often positioned on the boundary of properties, which places a limit on the footprint of the wall system.
Any of the wall types requiring reinforcement will not be suitable in such cases, as the reinforcing elements (grids or
straps) will encroach over the boundary.
Reinforced walls also frequently have to be bulked out and then re-compacted or bulked out and replaced with a
suitable fill material. This is often impractical for cut face applications — even when the face is not on a boundary.
Boulders may be used for retaining cut faces, but these structures require skilled operators and rigorous monitoring
during the construction process.
Crib walls are excellent for cut face applications as they are well-understood repeatable-engineered structures.
Reinforced walls are ideal for retaining fill material as there is no unnecessary bulking out of material and the
reinforcing elements are encapsulated in the fill.
3. The footprint of the wall
Crib walls can be built to great height — in excess of 10 metres, but being a gravity system, the higher the wall, the
thicker the base (footprint) must be. This makes them unsuitable for applications where vertical height is required and
space is limited.
Concrete sleeper walls can be used in either cut face, or fill applications. As they are vertical cantilever systems,
they typically have a narrow footprint (usually around 500mm). This makes them ideal for use on small lot housing
developments where the land use of blocks can be effectively maximised. Sleeper walls are used up to heights of
3m with a vertical face.
The table shows typical Wall heights and corresponding footprints for boulder walls, concrete crib walls and
concrete sleeper walls.
Wall height and footprint comparison
Wall
height/mm
Boulder walls
Concrete crib walls
Concrete sleeper walls
Base
Setback
Footprint
Base
Slope
Setback
Footprint
Base
Setback
Footprint
1000
800
550
1350
500
250
200
950
280
200
480
1500
1100
650
1750
700
375
200
1275
310
200
310
2000
1400
775
2175
1000
500
200
1700
350
200
550
2500
1750
900
2650
1000
625
200
1825
380
200
580
3000
2150
1025
3175
1000
750
200
1950
400
200
600
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4. The constructability of the wall system
This includes considerations such as:
✓✓ The wall environment (for example whether it is adjacent to water, the soil chemistry, salinity, etc)
✓✓ Accessibility of the area where the wall is to be built
✓✓ The size of the footprint available for the wall
✓✓ The required height and length of the wall
✓✓ The slope of the ground above the wall
✓✓ The slope of the ground at the bottom of the wall
✓✓ Any surcharge loads applied above the wall (including live loads such as those applied by roads, or dead loads
such as those applied by buildings)
✓✓ The availability of solid ground for a foundation
✓✓ Any services, trenches or pipelines near the base of the wall.
5. Adherence to retaining structures code and other legislation
The structure must comply with the AS4678, Retaining Structures Code, and any manufactured/fabricated
products must also comply with their respective codes (for example, structural concrete must fulfil the requirements
stipulated in AS3600).
6. Design life, durability and long term maintenance
The structure must be durable and meet the design life requirements stipulated in the relevant construction codes.
Long-term maintenance requirements must be taken into consideration and factored into the cost.
7. Aesthetics
This has become an increasingly important consideration in the selection, design and construction of retaining walls.
Modern walls can be planted out and/or coloured with long-lasting coloured sealants to enhance the aesthetic aspect.
Concrete sleeper walls have the advantage over timber walls as they can be constructed with a wood grain finish,
coloured with sealers, and they do not rot.
While these seven fundamental choice parameters serve as a useful guide when choosing a retaining wall, it pays to
bear in mind that every project is unique.
When finding the right solution to meet the specific challenges of YOUR project, it pays to consult the professionals.
At Concrib, we have more than 25 years of experience in the design, manufacture and construction of retaining walls
and earth stabilisation structures.
Call us on (07) 3375 1800, our team of industry-qualified technical consultants will gladly discuss your requirements
and advise you on the best solution for your project.
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